It should take part on every link building campaign you are about to run (lets say each 3 months, the first 3 months you have 2k inbound links and PR3 and look for similar sites as well, second part you have reached 3k inbound links and PR4 and again look for similar sites but don't deprecate the ones with slightly lower inbound links and pagerank).
From this type of webmasters I am putting you aware of people. They don't care about the quality, nor they will ever care to sell you something of quality, what they focus on is how to quickly make a site that will look good enough for and sell you out an advertisement spot. You are free to do so, as long as you have the time, money and energy to waste. But try not to waste other ones time by selling similar links.
Thaks for this article, it really is very informative. Now I found a place to look at all my questions concerning this topic.